1. Field of the Invention
The present invention relates to a method for producing a grain-oriented electrical steel sheet having improved magnetic properties.
2. Description of the Related Arts
The grain-oriented electrical steel sheet has a secondary recrystallized texture consisting of (110) [001]orientation which is easily magnetized in the rolling direction and is used as the core materials of a transformer, a power generator, or the like. The grain-oriented electrical steel sheet is industrially produced as follows. Molten steel having an appropriate composition is obtained by a converter process, an electric arc process, or the like. The molten steel is continuously cast to produce a slab. The slab is heated and then hot-rolled to produce a hot-rolled strip. The hot-rolled strip is pickled and occasionally annealed, and subsequently cold-rolled once or twice with an intermediate annealing to produce a cold-rolled strip having a final thickness. The cold-rolled strip is decarburization annealed and annealed at a satisfactorily high temperature, to induce the secondary recrystallization. In these sequential production steps the slab-heating step is important for dissolving the inhibitors, such as MnS, AlN and the like, predominant for the secondary recrystallization, and for preventing an abnormal growth of the continuously cast structure. The magnetic properties of the grain-oriented electrical steel sheet are, therefore, greatly influenced by the slab-heating step.
As is well known, the slabs for producing electrical steel sheets are heated at a temperature of from approximately 1200.degree. to 1400.degree. C.
Japanese Examined Patent Publication No. 56-18654 proposes, for preventing grain-coarsening of the slabs, and accordingly, improving the magnetic properties, to increase the heating rate by not less than 15.degree. C./hr in a high temperature range of slab-heating.
Japanese Unexamined Patent Publication No. 56-152926 proposes, also for preventing grain-coarsening of the slabs, to directly measure the slab-temperature by a thermocouple and to control the slab-heating, thereby attaining a heating temperature of 1300.degree. C. or more at the slab center and surface, and a soaking time of less than 70 minutes.